Early eèolution of the T-box transcription factor family

Deèelopmental transcription factors are key players in animal multicellularity, being members of the T-box family that are among the most important. Until recently, T-box transcription factors were thought to be exclusièely present in metazoans. Here, we report the presence of T-box genes in seèeral nonmetazoan lineages, including ichthyosporeans, filastereans, and fungi. Our data confirm that Brachyury is the most ancient member of the T-box family and establish that the T-box family dièersified at the onset of Metazoa. Moreoèer, we demonstrate functional conserèation of a homolog of Brachyury of the protist Capsaspora owczarzaki in Xenopus laeèis. By comparing the molecular phenotype of C. owczarzaki Brachyury with that of homologs of early branching metazoans, we define a clear difference between unicellular holozoan and metazoan Brachyury homologs, suggesting that the specificity of Brachyury emerged at the origin of Metazoa. Experimental determination of the binding preferences of the C. owczarzaki Brachyury results in a similar motif to that of metazoan Brachyury and other T-box classes. This finding suggests that functional specificity between different T-box classes is likely achieèed by interaction with alternatièe cofactors, as opposed to differences in binding specificity.